P0136: O2 Sensor Circuit (Bank 1 Sensor 2)
The engine control unit has detected a fault in the rear oxygen sensor circuit on bank 1.
Normal risk
Safe to drive. Have it checked at the next opportunity.
Short answer · Can I keep driving with P0136?
Yes. The rear sensor mainly checks the catalytic converter, so driving is not affected. Have it fixed before an emissions test.
What does P0136 mean?
Code P0136 is a general circuit fault for the oxygen sensor after the catalytic converter on bank 1. Without a valid rear signal, the control unit cannot test the converter.
What the car does when the code is set
The check engine light comes on. For a front sensor, the engine may stop using the sensor and run on a fixed mixture, which can raise fuel use and emissions. A rear sensor fault mostly affects the catalyst test.
How the system works
Oxygen sensors measure the oxygen left in the exhaust. The front sensor, before the catalytic converter, tells the engine control unit whether the mixture is rich or lean so it can correct the fuel. The rear sensor, after the converter, checks how well the converter works. The control unit watches the sensor voltage and current; if the signal stays too low, too high or does not change, it stores a circuit code.
Common symptoms
Possible causes
Listed without ranking. The cause on your car depends on make, engine and mileage — see the maker notes below.
- Failed oxygen sensorSensors age and can stop producing a proper signal.
- Wiring or connector damageSensor wires run near the hot exhaust and can melt, chafe or corrode.
- Exhaust leak near the sensorOutside air entering the exhaust makes the sensor read lean.
- ContaminationOil, coolant or silicone residues can poison the sensor element.
- Real mixture problemA genuinely rich or lean engine can push the signal to its limits.
How it is diagnosed
- Read live sensor dataWith the engine warm, a working front sensor switches rapidly or follows mixture changes; a stuck or flat signal points to the sensor or wiring.
- Inspect the wiring and connectorFollow the sensor wire for melted insulation, chafing or corrosion at the plug.
- Check for exhaust leaksLook and listen for leaks at the manifold and joints near the sensor.
- Fix fuel trim problems firstIf lean or rich codes are also stored, they may be the cause rather than the sensor.Let the exhaust cool before working near the sensor.
Questions about P0136
Does a rear oxygen sensor fault affect fuel use?
Usually only a little, because the front sensor controls the mixture. It mainly affects the catalyst test.
Does a new oxygen sensor always fix the code?
Not always. Wiring damage, exhaust leaks and real mixture faults cause the same codes, so check them before replacing the sensor.